The Ospreys saw off Cardiff Blues' stern effort to reclaim a Pro12 play-off place in Welsh rugby's Judgement Day at the Millennium Stadium.
Wales full-back Leigh Halfpenny scored the opening try for the Blues and also kicked 11 points in a 16-point haul.
But Ospreys hit back with tries from James King and Jonathan Spratt helping them into a 20-10 interval lead.
Dan Biggar kicked 13 points as Ospreys overhauled the Scarlets, who had earlier beaten Newport Gwent Dragons.
The second clash of the double-header had much to live up to in the wake of the day's initial east-west Welsh rugby clash.
There was a promising start for the 36,000-plus crowd to savour as the teams set about taking the game to each other.
Kahn Fotuali'i's lofted kick found Halfpenny, and although the full-back was swamped by chasers, Sam Warburton won the first of the breakdown battles with Wales rival and team-mate Justin Tipuric by stealing possession.
That launched the game's first telling attack in which centre Gavin Evans and Harry Robinson helped set Halfpenny free for the opening try.
Halfpenny converted the early score and Biggar hit back with the first of his penalties.
The Ospreys lost Ian Evans - replaced by Joe Bearman - after the Wales lock took a knock to a leg as the opening quarter drew to a close.
But they brushed off that setback to take the lead, Biggar and Richard Fussell combining before flanker King gave Halfpenny a delicious dummy on his way to the line.
Biggar converted but when Tipuric handled on the floor, Halfpenny struck back only for the Ospreys to take the initiative as the opening period wore on.
Biggar added a penalty and after Fussell made good ground, Ashley Beck sent Spratt over, Biggar adding the extras from the touchline.
Halfpenny kicked the second period's first points in the 53rd minute.
Ten minutes later, with the Blues having begun to edge the scrum battle against the Ospreys' renowned front row, Halfpenny added another penalty to leave them only four points behind.
Soon after, Biggar missed a chance to restore the seven-point advantage, sending his shot wide.
The Ospreys restored their supremacy at the scrum in the dying stages, but were denied when the ball squirted out as they approached the Blues' line.
After another set-piece, Biggar knocked on and opposite number Rhys Patchell offered brief relief with a 45-metre clearance.
Bearman gave the Blues a late chance after handling while offside, but Tipuric ended the contest with the final clearance kick.
Blues: Leigh Halfpenny; Owen Williams, Gavin Evans (capt), Dafydd Hewitt, Harry Robinson; Rhys Patchell, Lloyd Williams; Sam Hobbs, Marc Breeze, Scott Andrews, Filo Paulo, Lou Reed, Michael Paterson, Luke Hamilton, Sam Warburton.
Replacements: Kristian Dacey for Breeze (55), Thomas Davies, Taufa'ao Filise for Hobbs (41), Robin Copeland for Hamilton (41), Rory Watts-Jones, Lewis Jones for L Williams (63), Ceri Sweeney, Gareth Davies.
Not Used: Thomas Davies, Rory Watts-Jones, Ceri Sweeney, Gareth Davies.
Ospreys: Richard Fussell; Tom Habberfield, Jonathan Spratt, Ashley Beck, Tom Isaacs; Dan Biggar, Kahn Fotuali'i; Ryan Bevington, Richard Hibbard, Adam Jones, Alun Wyn Jones (capt), Ian Evans, James King, Jonathan Thomas Justin Tipuric.
Replacements: Ben John for Isaacs (58), Duncan Jones for Bevington (58), Scott Baldwin for Hibbard (65), Joe Bearman for Evans (17), Lloyd Peers for J Thomas (75).
Not Used: Dmitri Arhip, Rhys Webb, Matthew Morgan.
Referee: Nigel Owens (WRU, 95th competition game)
Assistant Referees: Ian Davies, Sean Brickell (both WRU)
Citing Commissioner: Denis Jones (WRU)
TMO: Nigel Whitehouse (WRU)